1. Field of the Invention
This invention relates to radio communication systems and methods, and more particularly relates to an interference cancellation system capable of providing improved interference suppression of wideband signals. Even more specifically, this invention relates to a signal path length correlator and method for use in an interference cancellation system for automatically calculating and adjusting for a time difference between the path length of an interference cancellation signal and a received interference signal to optimize suppression of the interference signal.
2. Description of the Prior Art
Operation of a receiver may be disrupted by interference signals generated from a collocated transmitter or a remote transmitter radiating within the same receiver passband. A collocated source usually interferes with the receiver due to the finite isolation between the transmit and receive antennas. Separating the transmitter and receiver in order to increase the isolation is oftentimes not possible, due to physical constraints such as in the case of an airborne platform. In the case of a remote transmitter, a stronger interference source will dominate the receiver and prevent the signal of interest from being detected. The solution to both problems of collocated and remote transmitters is to use an interference cancellation system.
An interference cancellation system takes a sample of an interference signal and adjusts the magnitude and phase such that the result is equal in amplitude and 180.degree. out of phase with the interference signal at the receiver. The vector sum of the two signals will cancel, leaving only the signal of interest. In practice, however, the two signals are not identical, due to unwanted distortion in the reference path and differences in signal path lengths. Cancellation performance is a function of amplitude and phase match between the interference signal and the sampled signal. To suppress a wideband interference signal, the performance of a cancellation system is directly proportional to the path length match between the cancellation signal and the interference signal. For a modulated carrier, phase match is determined by the signal bandwidth and the path length difference in time (often measured in nanoseconds) between the transmit antenna to the receive antenna signal path and the sampled reference signal path to the receive antenna.
In order to provide good signal suppression, the cancellation signal path length should be adjusted to time match the interference signal path length. Contemporary techniques for this adjustment involve manual measurements to determine the path length difference between the interference cancellation signal and the received interference signal. More specifically, the presently used method of time matching the cancellation signal path and the interference signal path employs external equipment to observe the cancelled waveform while adjusting the cable length (i.e., the electrical length) of the cancellation signal path to achieve maximum cancellation, which is synonymous with good time match. This procedure is performed only once during installation and requires specially trained personnel. The same measurement must be repeated whenever the system configuration is changed. The conventional method does not lend itself to situations where the time match may change over the course of time as a result of a changing environment. Thus, a conventional interference cancellation system may suffer a loss of suppression performance due to system dynamics.